Liquid-developing electrophotographic apparatus

ABSTRACT

A liquid-development electrophotographic apparatus of the present invention uses a nonvolatile liquid developer. An electric field force causes toner to adhere to an electrostatic latent image formed on a photoconductive member  2  to thereby form a toner image on the photoconductive member  2 . Viscoelasticity control means is provided for controlling the viscoelasticity of a toner image transferred from the photoconductive member  2  onto an intermediate transfer member  3 . A temperature of the liquid toner at which a predetermined requirement for a dynamic viscoelastic value is satisfied is obtained beforehand by preliminary measurement. The viscoelasticity control means controls heating by a heater  4 , which serves as heating means, in such a manner that the toner image on the intermediate transfer member  3  is heated to the temperature before being transferred onto a printing medium  6 . A carrier-agent-removing roller  7  of reverse rotation is provided for removing a carrier agent from the toner image whose viscoelasticity has been controlled.

TECHNICAL FIELD

The present invention relates to a liquid-developmentelectrophotographic apparatus that uses a liquid developer, and providesa liquid-development electrophotographic apparatus that, when a melttransfer process is employed for transferring a toner image formed on animage-bearing member, such as a photoconductive member or anintermediate transfer member, to a printing medium, can print with highimage quality by controlling the viscosity of the liquid developer to anoptimum viscoelastic characteristic value without need to apply anexcessively high pressure in the course of transfer of the toner imageonto the printing medium.

BACKGROUND ART

An electrophotographic apparatus that uses a liquid developer hasemployed an electrostatic transfer process for transferring a tonerimage onto a printing medium, such as paper. In the electrostatictransfer process, bias voltage is applied to a toner image formed on animage-bearing member to thereby transfer the toner onto the printingmedium. However, since such electrostatic transfer is influenced by theelectric resistance of the printing medium, printing quality greatlydepends on environmental conditions, such as temperature and humidity.Accordingly, the environmental specifications of a printer systeminclude restrictive environmental conditions.

In order to solve such a problem, a melt transfer process is proposed.In the melt transfer process, before transfer of a toner image onto aprinting medium, toner particles (solid component) are melted byapplication of heat; and the adhesive force of the molten toner solidcomponent is utilized for transfer onto the printing medium.

As shown in FIG. 12, according to the conventional process, developingunits 51 corresponding to a plurality of colors cause, by utilization ofelectric field force, toner particles in the corresponding colors toadhere to corresponding electrostatic latent images formed on aphotoconductive member 50, thereby forming a toner image on thephotoconductive member 50. Before the toner image is transferred onto aprinting medium 53, the toner image particles are melted by applicationof heat from a heater 54 contained in the photoconductive member 50. Ina transfer section, a backup roller 52 causes the molten toner imageparticles to be pressed against the printing medium 53 for transfer ontothe printing medium 53.

In the case where such a melt transfer process uses a volatile liquiddeveloper, sufficient adhesive force required for transfer of a tonerimage onto a printing medium can be secured without weakening of thecohesive force of the molten toner image particles, since a carrieragent contained in the liquid developer volatilizes before transfer ofthe toner image. However, in the case where such a volatile liquiddeveloper is used, a large-scaled volatile-solvent collection systemmust be employed in order to prevent a volatilized carrier agent fromaffecting the body of a user of the electrophotographic apparatus.

In the case where a nonvolatile liquid developer is used, a carrieragent contained in the liquid developer weakens the cohesive force ofthe molten toner image particles. In order to cope with the problem, asshown in FIG. 2(C), the toner particles are completely melted into aliquefied condition to thereby forcibly expel the carrier agent frominter-toner-particle spacing for removal. However, as a result of thetoner particles being melted, an adhesive force generated by the tonerparticles themselves fails to be sufficiently utilized for transfer,resulting in a failure to secure sufficient adhesive force required fortransfer of a toner image onto a printing medium. Thus, in order tocompensate for a weakened cohesive force of toner image particles forobtaining sufficient adhesive force, for use in a conventionalelectrophotographic apparatus that uses a nonvolatile liquid developerand employs a melt transfer process, there is proposed an apparatus inwhich a backup roller applies excessively high pressure in the course oftransfer of the toner image onto a printing medium (refer to, forexample, Japanese Patent Application Laid-Open (kokai) No. 2002-311725).

However, in some cases, such an apparatus in which the backup rollerapplies excessively high pressure in the course of transfer involves thefollowing problem: when a printing medium is fed into a contact regionbetween an image-bearing member and the backup roller, vibration isgenerated in the apparatus, thereby causing generation of an image noisecalled a “shock mark” and thus hindering printing with high imagequality.

DISCLOSURE OF THE INVENTION

As described above, the prior art techniques involve the followingproblem.

An electrophotographic apparatus that uses a liquid developer hasemployed an electrostatic transfer process, in which electric fieldforce is applied so as to cause the movement of toner particles toward aprinting medium in the course of transfer of a toner image from animage-bearing member onto the printing medium. However, theelectrostatic transfer is apt to involve a defective transfer onto theprinting medium, depending on working environmental conditions,particularly working temperature and humidity, of theelectrophotographic apparatus, resulting in a hindrance to printing withhigh image quality.

In order to solve the above problem, a melt transfer process isproposed. In the melt transfer process, toner particles, which are asolid component contained in the liquid developer, are melted byapplication of heat so as to utilize the adhesive force of the tonerparticles themselves for transfer onto a printing medium. However, inthe case of an electrophotographic apparatus using a nonvolatile liquiddeveloper, even when toner particles, which are a solid componentcontained in the liquid developer, are melted, a carrier agent, which isa liquid component contained in the liquid developer, weakens thecohesive force of the toner image particles; as a result, in some cases,an adhesive force generated as a result of the toner particles beingmelted is insufficient for satisfactory transfer of the toner image ontoa printing medium.

In order to solve the above problem, there is proposed an apparatus inwhich, in the course of transfer of a toner image onto a printingmedium, a backup roller applies excessively high pressure so as tocompensate for a carrier-agent-weakened cohesive force of toner imageparticles. However, such an apparatus in which excessively high pressureis applied in the course of transfer of a toner image onto a printingmedium involves the following problem: when the printing medium is fedinto an image transfer section, vibration is generated in the apparatus,thereby causing generation of noise called a “shock mark” and thushindering image quality.

An object of the present invention is to provide an electrophotographicapparatus that uses a nonvolatile liquid developer and can completelytransfer a toner image onto a printing medium, without need to apply anexcessively high pressure, by use of a melt transfer process, in whichtoner particles, which are a solid component contained in the liquiddeveloper, are melted for transfer onto the printing medium, therebyenabling printing with high image quality free from generation of noise,such as a shock mark.

A liquid-development electrophotographic apparatus of the presentinvention performs transfer in such a manner that a toner image formedby developing a formed electrostatic latent image by use of anonvolatile liquid developer is transferred from an image-bearing memberonto a printing medium by a melt transfer process. Theliquid-development electrophotographic apparatus comprises control meansfor controlling the viscoelasticity of a toner image on theimage-bearing member by bonding toner particles of the toner imagetogether by means of partially melting the toner particles, so as tocause the liquid toner to enter a liquid-toner-softened condition havinga carrier agent in inter-bonded-toner-particle spacing. The controlmeans causes the toner particles to be bonded together without causingthe toner particles to be melted to such an extent as to be liquefied,and causes the bonded toner particles to be separated from the carrieragent. The liquid-development electrophotographic apparatus furthercomprises carrier-agent-removing means for removing the carrier agentfrom the viscoelasticity-controlled toner image. Thecarrier-agent-removing means has a surface in contact with the carrieragent caused to float by use of electric field force, and removes thecarrier agent by moving the surface in a direction opposite a movingdirection of the toner image.

The viscoelasticity of the toner image is controlled such that, when thedynamic viscoelasticity of the toner image is measured at a forcedvibration frequency of 1 Hz and an amplitude stress of 10 Pa, a storagemodulus falls within a range of 1.0E5 Pa to 1.0E8 Pa, and a loss modulusfalls within a range of 1.0E5 Pa to 1.0E8 Pa.

A temperature of the liquid toner at which the above-mentionedrequirement for a dynamic viscoelastic value is satisfied is obtainedbeforehand by preliminary measurement. The viscoelasticity control meanscan assume the form of means for heating the toner image on theimage-bearing member to the obtained temperature.

A carrier agent can be removed immediately before transfer of the tonerimage onto a printing medium as described below. While control isperformed so as to satisfy the above-mentioned requirement for thedynamic viscoelasticity of the toner image on the image-bearing member,bias voltage having the same polarity as that of a charge established onthe toner is applied to the toner image so as to impose electric fieldforce on the toner in such a direction as to press the toner against theimage-bearing member, thereby causing the carrier agent to float on thetoner. The floating carrier agent is removed by means of a moving memberthat moves at a speed equal to or higher than the moving speed of thetoner image on the image-bearing member in a direction opposite themoving direction of the toner image. Furthermore, within 2,000 ms afterthe removal of the floating carrier agent, the toner image istransferred onto the printing medium.

In order to maintain the condition in which the above-mentionedrequirement for the dynamic viscoelasticity of the toner image issatisfied, heating the toner image on the image-bearing member by theheating means may be controlled in such a manner that the temperature ofthe image-bearing member becomes not higher than the boiling point ofthe carrier agent and not higher than 100° C.

Preferably, while the above-mentioned requirement for the dynamicviscoelasticity of the toner image is satisfied, the carrier agent isremoved in such a manner that, before the toner image is transferredonto the printing medium, the solid content of the toner image on theimage-bearing member is 50% to 95%.

Preferably, while the above-mentioned requirement for the dynamicviscoelasticity of the toner image is satisfied, a pressure to beapplied in the course of transfer of the toner image onto the printingmedium is controlled to 0.5 MPa to 4.0 MPa.

In the case of color printing in which toner images whose dynamicviscoelasticity satisfies the above-mentioned requirement are superposedon each other on the image-bearing member, the carrier agent can beremoved each time a toner image in each of a plurality of colors istransferred onto the image-bearing member, by means of a moving memberthat moves at a speed equal to the moving speed of the image-bearingmember in the same direction as the moving direction of theimage-bearing member.

When the toner image whose dynamic viscoelasticity satisfies theabove-mentioned requirement is to be transferred onto the printingmedium, the printing medium can be heated beforehand to not higher than[(the lowest temperature at which the dynamic viscoelasticity is suchthat the storage modulus is 1.0E5 Pa or less, and the loss modulus is1.0E5 Pa or less)+50° C.].

When the toner image whose dynamic viscoelasticity satisfies theabove-mentioned requirement is to be transferred onto the printingmedium, bias voltage can be applied in such a direction as to cause thetoner image to move toward the printing medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view exemplifying an electrophotographic apparatus using anonvolatile liquid developer and transferring a toner image onto aprinting medium by a melt transfer process;

FIG. 2 is a series of explanatory views showing a liquid toner indifferent molten conditions;

FIG. 3 is a pair of explanatory views showing a process for removingunnecessary carrier agent;

FIG. 4 is an explanatory view showing the relationship between acarrier-agent-removing process and a position of transfer onto aprinting medium;

FIG. 5 is an explanatory view showing re-dispersion of toner solid afterremoval of a carrier agent;

FIG. 6 is a pair of explanatory views showing a process for removing acarrier agent each time a color toner image in each color istransferred;

FIG. 7 a pair of explanatory views showing the effect of acarrier-agent-removing process in color printing;

FIG. 8 is an explanatory view showing application of pressure by abackup roller in the course of transfer onto a printing medium;

FIG. 9 is a configurational view of an apparatus having means forheating a printing medium before transfer onto the printing medium;

FIG. 10 is an explanatory view showing a process for moving toner towarda printing medium by means of bias voltage;

FIG. 11 is a pair of explanatory views showing a material for theoutermost surface of an intermediate transfer member;

FIG. 12 is an explanatory view showing a conventional melt transferprocess;

FIG. 13 is a pair of explanatory views showing the conditions oftransfer of a toner image onto a printing medium in a melt transferprocess; and

FIG. 14 is a table showing numerical values used to draw the graph ofFIG. 13(B).

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will next be described with reference to anembodiment. In the below description, similar features are denoted bycommon reference numerals, and repeated description thereof may beomitted. An electrophotographic apparatus that uses a nonvolatile liquiddeveloper employs an electrostatic transfer process, which is influencedby environmental factors, or a melt transfer process, which is notinfluenced by environmental factors.

FIG. 1 exemplifies an electrophotographic apparatus that uses anonvolatile liquid developer and employs a melt transfer process fortransfer of a toner image onto a printing medium. In the example of FIG.1, a photoconductive member 2 is provided for each of a plurality ofcolors and assumes the form of a drum having an insulator film whoseelectric resistance drops upon exposure. Each of the photoconductivemembers 2 is provided with, for example, a charger (not shown) forcharging the photoconductive member 2, exposure means (not shown), suchas LED, for exposing the photoconductive member 2, and a developing unit1. The individual exposure means cause the formation of correspondingelectrostatic latent images on the corresponding photoconductive members2 with respect to a plurality of colors (for example, yellow, magenta,cyan, and black). The developing units 1 corresponding to the colorscause toner particles to adhere to the corresponding electrostaticlatent images by use of electric field force, thereby forming tonerimages in the colors. The toner images in the colors are transferredonto an intermediate transfer member 3 and superposed on each other. Atoner image resulting from the transferred toner images being superposedon each other is melted to a condition most suited for transfer by meansof heating the toner image to a predetermined temperature by use of aheater 4 contained in the intermediate transfer member 3. The tonerimage in the molten condition is transferred onto a printing medium 6,such as paper, under subjection to pressure applied by a backup roller5. The electrophotographic apparatus is configured and operates as doesan ordinary electrophotographic apparatus.

As will be described in detail later, according to the configurationshown in FIG. 1, a carrier-agent-removing roller 7 of reverse rotationis provided on the intermediate transfer member 3 at a position locatedimmediately before a position of transfer onto the printing medium 6;and carrier-agent-removing-rollers 8 of forward rotation are provided onthe intermediate transfer member 3 at positions downstream of thecorresponding photoconductive members 2.

The configuration shown in FIG. 1 includes viscoelasticity controlmeans, which is a feature of the present invention, and temperaturecontrol means, which is controlled by the viscoelasticity control meansand controls the temperature of a heater 4. The heater contained in theintermediate transfer member heats the intermediate transfer member tothereby heat a toner image formed on the surface of the intermediatetransfer member.

The illustrated apparatus uses nonvolatile liquid developers and causestoners to adhere to corresponding electrostatic latent images formed onthe corresponding photoconductive members by means of electric fieldforce, thereby forming toner images on the corresponding photoconductivemembers. The viscoelasticity control means controls the viscoelasticityof a toner image formed of the toner images transferred from thephotoconductive members onto the intermediate transfer member. Theviscoelasticity is controlled such that, when the dynamicviscoelasticity of the toner image is measured at a forced vibrationfrequency of 1 Hz and an amplitude stress of 10 Pa, a storage modulusfalls within a range of 1.0E5 Pa to 1.0E8 Pa, and a loss modulus fallswithin a range of 1.0E5 Pa to 1.0E8 Pa (E represents the power of 10;thus, E5=10⁵, and E8=10⁸). The below description refers to an example ofcontrolling the heating of the intermediate transfer member forviscoelasticity control. However, the viscoelasticity of toner imageparticles can also be controlled by, in addition to temperature control,adjusting a carrier removal percentage as measured before transfer ontothe printing medium or adjusting electric field force to be applied inthe course of transfer onto the printing medium.

In the illustrated apparatus, a temperature of the liquid toner at whichthe above-mentioned requirement for a dynamic viscoelastic value issatisfied is obtained beforehand by preliminary measurement; and theviscoelasticity control means controls heating by the heater 4, whichserves as heating means, in such a manner that the toner image on theintermediate transfer member (image-bearing member) is heated to thetemperature before being transferred onto the printing medium.

The above heating temperature is controlled desirably such that thetemperature of the image-bearing member (intermediate transfer member inthe illustrated example) becomes not higher than the boiling point ofthe carrier agent and not higher than 100° C. This is to prevent thefollowing problems: even the carrier agent that is nonvolatile at theroom temperature volatilizes when the temperature of the image-bearingmember becomes the boiling point of the carrier agent or higher, therebyaffecting the human body; and when the image-bearing member is heated toan excessively high temperature, the image-bearing member is thermallydamaged to thereby be deteriorated.

The heating of the image-bearing member is controlled as mentioned aboveto thereby heat the toner image on the image-bearing member to apredetermined temperature, whereby the above-mentioned requirement for adynamic viscoelastic value is satisfied. Additionally, as will bedescribed in detail later, the carrier agent is removed from the tonerimage by means of the carrier-agent-removing roller 7 of reverserotation immediately before the toner image is transferred onto theprinting medium.

FIG. 13 is a pair of explanatory views showing the conditions of thetransfer of the toner image onto the printing medium in a melt transferprocess. FIG. 13(A) is a view explaining forces F1, F2, and F3; and FIG.13(B) is a graph showing the results of measurement with respect to therelationship between transfer and the viscoelasticity (storage modulusor loss modulus) of toner particles. In FIG. 13(B), the horizontal axisrepresents the viscoelasticity of toner particles; and the vertical axisrepresents a generated force. Notably, storage modulus or loss modulusrepresented along the horizontal axis of the graph numerically decreasesas melting progresses. Strictly, storage modulus and loss modulus mustbe represented separately. However, since they exhibited substantiallythe same tendency, the graph was drawn in a simplified manner. FIG. 14is a table that shows numerical values used to draw the graph of FIG.13(B) and numerically shows the relationship between storage modulus orloss modulus and the generated forces (F1, F2, and F3) acting on tonerparticles.

As shown in FIG. 13(A), when F1 represents a toner-image-holding force,F2 represents a cohesive force of toner image particles, and F3represents an adhesive force for adhesion of the toner image onto theprinting medium, the toner image can be transferred by 100% onto theprinting medium if the relation F2>F3>F1 holds.

As is apparent from the results of measurement shown in FIG. 13(B), F1and F3 increase as melting progresses. By contrast, F2 is a selfcohesive force and increases for unification until melting progresses toa certain extent. However, F2 begins to decrease when melting progressesexcessively. The relation F2>F3>F1 holds in the following case: when thedynamic viscoelasticity of the toner image transferred onto theimage-bearing member is measured at a forced vibration frequency of 1 Hzand an amplitude stress of 10 Pa, both of storage modulus and lossmodulus fall in a range of 1.0E8 Pa to 1.0E5 Pa. At this time, virtually100% of the toner image can be transferred onto the printing medium. Theviscoelasticity of toner particles represents the degree of thesoftening of toner particles (a resin component contained in the tonerimage) associated with the progress of the melting of toner particlesand is influenced not only by the temperature of the toner image butalso by the quantity of a carrier agent contained in the toner image.The degree of the softening of toner particles in the100%-transfer-enabled region can be represented by the above-mentionednumerical values. More specifically, 100% of the toner image can betransferred when the toner particles of the toner image are in asemi-molten condition (may also be called a “liquid-toner-softenedcondition,” in which the toner particles are partially melted and arenot melted to such an extent as to reach a completely molten condition.

FIG. 2 is a series of views for explaining the molten conditions of theliquid toner. FIG. 2(A) shows a toner-particle-unmolten condition inwhich toner particles are dispersed in a carrier agent. The nonvolatileliquid developer employed in the present apparatus uses a nonvolatilesilicone oil as a carrier agent. The viscosity of the silicone oil is 10cSt to 200 cSt, preferably 50 cSt to 100 cSt. Toner particles formedfrom resin and pigment and having a size of about 1 μm to 2 μm aredispersed in the silicone oil at a percentage of 10% to 30%, preferably10% to 20%. Notably, herein, the term “liquid toner” refers to acombined entity of a carrier agent and toner particles; and the term“toner image” refers to an assembly of toner particles in the form of animage.

FIG. 2(C) shows a completely molten condition of toner particles in aconventional melt transfer process. In the case where a nonvolatileliquid developer is used, a carrier agent, which is a carrier componentof the liquid developer and is a dispersant, remains in a toner imageeven at the time of transfer of the toner image onto a printing medium,since the carrier agent is nonvolatile. When the carrier agent remainsin a large quantity, the cohesive force F2 of toner image particles isweakened. Thus, before transfer, the carrier agent must be removed fromthe toner image to the greatest possible extent. In the conventionalmelt transfer process, as shown in FIG. 2(C), toner particles, which area solid component of the liquid developer, are completely melted andunified. As a result, the carrier agent that is present ininter-toner-particle spacing and cannot be expelled by use of electricfield force is forcibly expelled for removal.

As in the case of the conventional melt transfer process, it is knownthat melting of toner particles allows efficient removal of the carrieragent, which hinders transfer of the toner image onto the printingmedium. However, in the conventional melt transfer process, as a resultof toner particles being completely melted and unified, an adhesiveforce that causes the molten toner particles to adhere to animage-bearing member is generated. As a result of the molten tonerparticles sticking to the image-bearing member, the toner-image-holdingforce F1 of the image-bearing member increases. As a result, in somecases, the efficiency of transfer of the toner image onto the printingmedium drops; and, after transfer, difficulty is involved in cleaningoff residual toner particles from the image-bearing member.

FIG. 2(B) shows a liquid-toner-softened condition in the melt transferof the present invention. In contrast to the conventional melt transferin which toner particles are completely melted into a liquefiedcondition, according to the present invention, toner particles arepartially melted and bonded into a liquid-toner-softened condition inwhich a carrier agent is present in inter-toner-particle spacing. Thus,the toner particles, which are a solid component of the liquiddeveloper, are partially melted to thereby be softened, and are bondedto each other, whereby the toner particles are separated from thecarrier agent, which is a liquid component of the liquid developer, andthe removal of the carrier agent is facilitated. Accordingly, virtually100% of the toner image can be transferred onto the printing medium.

As has been described with reference to FIG. 13(B), theliquid-toner-softened condition is the semi-molten condition in whichtoner particles are partially melted, but are not completely melted. Theliquid-toner-softened condition corresponds to a viscoelastic range inwhich, when the viscoelasticity of the toner particles is measured underthe above-mentioned conditions, both of storage modulus and loss modulusfall within a range of 1.0E5 Pa to 1.0E8 Pa.

As in the case of the toner-particle-liquefied condition in theconventional melt transfer, the liquid toner in a softened condition inthe melt transfer of the present invention allows the carrier agentpresent in inter-toner-particle spacing to be removed satisfactorily.Additionally, since the toner particles are not melted to anunnecessarily intensive degree, the molten toner particles do not stickto the image-bearing member, so that the efficiency of transfer of thetoner image onto the printing medium do not drop.

As mentioned above, the carrier-agent-removing roller 7 of reverserotation shown in FIG. 1 is adapted to remove the carrier agent from theliquid toner that is in a softened condition and includes the carrieragent in inter-toner-particle spacing. The carrier-agent-removing roller7 of reverse rotation will be further described with reference to FIG.3. FIG. 3 is a pair of explanatory views showing a process for removingan unnecessary carrier agent. FIG. 3(A) shows a process for causing thecarrier agent to float up; and FIG. 3(B) shows a process for removingthe floating carrier agent. As shown in FIG. 3(A), bias voltage havingthe same polarity as that of a charge established on the toner particlesis applied between the intermediate transfer member 3 and thecarrier-agent-removing roller 7 of reverse rotation, thereby imposingelectric field force on the charged toner particles. As a result, thetoner particles are pressed against the surface of the intermediatetransfer member 3, whereas the carrier agent is caused to float on thetoner particles.

As shown in FIG. 3(B), the carrier-agent-removing roller 7 of reverserotation is in contact with the floating carrier agent and removes thefloating carrier agent. At this time, the carrier-agent-removing roller7 of reverse rotation is caused to rotate in such a manner that itssurface and the surface of the intermediate transfer member 3, which isin contact with the surface of the carrier-agent-removing roller 7 ofreverse rotation via the liquid toner, move in opposite directions (thisis herein called “reverse rotation”).

The carrier-agent-removing roller 7 of reverse rotation, which is causedto move in a direction opposite that in which the intermediate transfermember 3 moves, is rotated at such a rotational speed that its surfacein the contact region moves at a speed equal to or higher than themoving speed of the toner image on the intermediate transfer member. Byuse of the carrier-agent-removing roller 7 of reverse rotation, anunnecessary carrier agent is removed. Thus, the carrier agent that iscaused to float on the toner image by the above-mentioned electric fieldforce can be removed almost completely. Such carrier removal isperformed at least once on the toner image that is formed by superposingtoner images in a plurality of colors on each other, immediately beforethe toner image is transferred onto the printing medium. The carrieragent that has moved from the intermediate transfer member 3 to thecarrier-agent-removing roller 7 can be removed by use of, for example, ablade in contact with the surface of the carrier-agent-removing roller7.

When the carrier agent floating on the toner image is removed, thecarrier agent, which weakens an adhesive force for adhesion to theprinting medium, is absent on the surface of the toner image having anadhesive force generated as a result of being melted. Thus, the adhesiveforce for adhesion to the printing medium is increased, thereby enablingconsistent transfer.

As shown in FIG. 4, the toner image is transferred onto the printingmedium 6 within 2,000 ms after the carrier agent is removed by means ofthe carrier-agent-removing roller 7 of reverse rotation. In other words,the removal of the carrier agent is performed on the intermediatetransfer member 3 at a position located immediately before the positionof transfer.

The reason for the above-mentioned removal of the carrier agentimmediately before transfer is as follows. As shown in FIG. 5, when acertain time elapses after the carrier agent floating on the toner imageis removed by means of the carrier-agent-removing roller 7 of reverserotation before transfer of the toner image onto the printing medium, aresidual carrier agent causes the bonded toner particles to re-disperse.When the toner particles re-disperses, as time elapses, the carrieragent remaining in the toner image floats again on the toner image. As aresult, the adhesive force F3 of the toner image for adhesion to theprinting medium weakens, and the cohesive force F2 of toner imageparticles weakens, thereby hindering consistent transfer onto theprinting medium.

When the toner image is to be transferred onto the printing medium afterthe carrier agent is removed in the liquid-toner-softened condition, inwhich the above-mentioned requirement for a dynamic viscoelastic valueis satisfied, by means of the carrier-agent-removing roller 7 of reverserotation, the solid content of the toner image on the intermediatetransfer member is preferably rendered 50% to 95%. The carrier agentcontained in the toner image weakens the cohesive force F2 of tonerimage particles at the time of transfer onto the printing medium,thereby hindering the transfer. Therefore, removing the carrier agent tothe greatest possible extent is desirable. However, when the carrieragent is removed almost completely, the toner image is stuck to theimage-bearing member, possibly resulting in a drop in the efficiency oftransfer. Also, after transfer, difficulty may be involved in cleaningoff residual toner from the image-bearing member. Thus, by means ofremoving the carrier agent contained in the toner image while theabove-mentioned toner content range, which allows consistent transfer,is maintained, the toner image can be reliably transferred onto theprinting medium, and the residual toner can be readily cleaned off fromthe image-bearing member.

FIG. 6 is a pair of explanatory views showing a process for removing thecarrier agent each time a toner image in each color is transferred. Ashas been described with reference to FIG. 1, in color printing, tonerimages in basic colors, such as yellow, magenta, cyan, and black, aresuperposed on each other on the intermediate transfer member so as toform a color toner image; and the color toner image is transferred ontothe printing medium for printing. The carrier-agent-removing-rollers 8of forward rotation are provided on the intermediate transfer member 3at positions downstream of the corresponding photoconductive members 2so as to remove the carrier agent each time the corresponding tonerimages in colors are transferred.

As shown in FIG. 6(A), for example, after a toner image in magenta,which is the first color, is transferred onto the intermediate transfermember 3, the carrier agent is removed; next, as shown in FIG. 6(B),after a toner image in yellow, which is the second color, is transferredonto the intermediate transfer member 3 in such a manner as to besuperposed on the toner image in magenta, the carrier agent is removed.In this manner, each time a toner image in each color is transferredonto the intermediate transfer member 3, the carrier agent is removed.For the removal of the carrier agent to be performed each time a tonerimage in each color is transferred, the carrier-agent-removing-rollers 8of forward rotation are provided. Each of thecarrier-agent-removing-rollers 8 of forward rotation rotates in the samedirection as the moving direction of the toner image on the intermediatetransfer member 3 at such a rotational speed that both surfaces in thecontact region move at the same speed. If a stationary blade or the likeis used to remove an excess carrier agent, shearing force generated inassociation with the removal of the carrier agent may disturb the tonerimage, potentially resulting in an impairment in image quality. Byvirtue of using the above-described carrier-agent-removing-rollers 8 offorward rotation, generation of shearing force can be prevented in thecourse of the removal of the carrier agent. Therefore, the carrier agentcan be removed without involvement of a disturbance of the toner image.

FIG. 7 is a pair of explanatory views showing the effect of thecarrier-agent-removing process in color printing. A toner image in eachcolor contains the carrier agent. After a toner image in a certain coloris transferred onto the intermediate transfer member 3, if a toner imagein the next color is superposed on the previous toner image withoutremoval of the carrier agent, as shown in FIG. 7(A), the carrier agentis sandwiched between the toner images in colors. When the resultanttoner image having inner carrier-agent layers formed therein is to betransferred onto the printing medium, removal of an excess carrier agentbefore transfer becomes difficult. The residual carrier agent tends todisturb toner images in colors, potentially resulting in an impairmentin image quality.

Thus, by means of removing an excess carrier agent each time a tonerimage in each color is transferred onto the intermediate transfer member3, a final color toner image resulting from the transferred toner imagesin colors being superposed on each other is free of excess remainingcarrier agent, as shown in FIG. 7(B). Thus, an impairment in imagequality, which could otherwise result from a disturbance of the tonerimage, can be prevented.

FIG. 8 is an explanatory view showing application of pressure by thebackup roller in the course of transfer onto the printing medium. Asshown in FIG. 8, in a transfer section where the toner image istransferred onto the printing medium 6, the backup roller 5 appliespressure. Preferably, the pressure at the time of transfer is controlledto be 0.5 MPa to 4.0 MPa. In the case where a nonvolatile liquiddeveloper is used, application of pressure by use of the backup roller 5in the course of transfer of the toner image can compensate for lack ofthe cohesive force F2 of toner image particles caused by the carrieragent. Thus, there can be maintained the relation required forconsistent transfer “F2 (cohesive force of toner image particles)>F3(adhesive force for adhesion to printing medium)>F1 (toner-image-holdingforce of image-bearing member).”

In the case of transfer in the conventional melt transfer process, thebackup roller must apply excessively high pressure (4.0 MPa or greater)in order to compensate for the weakening of F2 (cohesive force of tonerimage particles) and F3 (adhesive force for adhesion to printing medium)caused by the carrier agent. This causes generation of vibration whenthe printing medium enters the transfer section, resulting in imagenoise. However, in the apparatus based on the present invention, thedynamic viscoelasticity of the toner image is controlled so as toestablish the liquid-toner-softened condition, which is most suited fortransfer. Thus, pressure to be applied in the transfer section can beset low, thereby preventing generation of image noise.

FIG. 9 is a configurational view of an apparatus having means forheating the printing medium before transfer onto the printing medium. Asshown in FIG. 9, the printing medium onto which the toner image is to betransferred is heated beforehand by means of a pair ofprinting-medium-heating rollers 9. Preferably, the printing medium isheated to a temperature not lower than the temperature of theintermediate transfer member 3 (image-bearing member) and not higherthan [(the lowest temperature at which the dynamic viscoelasticity ofthe toner image is such that the storage modulus is 1.0E5 Pa or less,and the loss modulus is 1.0E5 Pa or less)+50° C.]. Before the tonerimage is transferred onto the printing medium, the toner image is heatedso as to assume a dynamic viscoelastic value most suited for transfer.However, when the toner image comes into contact with the printingmedium, the temperature of the printing medium causes the temperature ofthe toner image to change, potentially causing the temperature of thetoner image to fall outside a temperature range for assuming a dynamicviscoelastic value most suited for transfer. As a result, in some cases,consistent transfer may be hindered.

In order to cope with the above problem, the printing medium is heatedbeforehand such that, when the toner image comes into contact with theprinting medium for transfer, the temperature of the toner image fallswithin a temperature range for assuming a dynamic viscoelastic valuemost suited for transfer.

FIG. 10 is the explanatory view showing a process for moving tonertoward the printing medium by means of bias voltage. In FIG. 10, aregion encircled by the broken line is shown below in an enlargedcondition. As shown in FIG. 10, when the toner image in theliquid-toner-softened condition, in which the toner image assumes adynamic viscoelastic value most suited for transfer, is transferred ontothe printing medium, bias voltage can be applied in such a direction asto move the toner image toward the printing medium. As described above,by means of causing the toner image to assume a dynamic viscoelasticvalue most suited for transfer, while a cohesive force of toner imageparticles and an adhesive force for adhesion to the printing medium aremaintained at respective levels required for transfer, electric fieldforce can be applied in such a direction as to move the toner imagetoward the printing medium. As a result, the toner-image-holding forceof the intermediate transfer member 3 (image-bearing member) can beweakened, whereby the relation required for complete transfer “F2(cohesive force of toner image particles)>F3 (adhesive force foradhesion to printing medium)>F1 (toner-image-holding force ofimage-bearing member)” can be reliably maintained.

In relation to the above-mentioned application of bias voltage, theelectric resistance of the intermediate transfer member 3 is preferably1.0E7 Ωcm to 1.0E10 Ωcm. In order to generate electric field force formoving the toner image on the intermediate transfer member 3 toward theprinting medium, the intermediate transfer member 3 must have anelectric resistance that falls within the above range. When the electricresistance of the intermediate transfer member 3 is too low, currentflows to a portion of the intermediate transfer member 3 other than thetoner image; therefore, in some cases, voltage is not applied to thetoner image, resulting in a failure to generate sufficient electricfield force. When the electric resistance of the intermediate transfermember 3 is too high, a voltage drop occurs on the intermediate transfermember 3; therefore, in some cases, sufficient voltage is not applied tothe toner image, resulting in a failure to generate sufficient electricfield force.

Thus, by means of setting the electric resistance of the intermediatetransfer member 3 to the above-mentioned range, voltage is effectivelyapplied to the toner image to thereby generate sufficient electric fieldforce for transfer, so that transfer can be consistently performed.

FIG. 11 is a pair of explanatory views showing a material for theoutermost surface of the intermediate transfer member. FIG. 11(A) showsthe intermediate transfer member 3 in contact with the backup roller 5;and FIG. 11(B) shows the contact region in an enlarged condition.Preferably, a rubber material of JIS-A 10 degrees to 80 degreesexhibiting high toner releasability is used to form the outermostsurface of the intermediate transfer member 3, from which the tonerimage in the liquid-toner-softened condition, in which the toner imageassumes a dynamic viscoelastic value most suited for transfer, istransferred onto the printing medium by use of the backup roller 5. Useof the material having high toner releasability can weakens thetoner-image-holding force of the intermediate transfer member 3. Asshown in FIG. 11(B), when pressure is applied from the backup roller 5at the time of transfer, use of the rubber material allows deformationof the transfer section to thereby increase a contact area with theprinting medium, thereby facilitating transfer and enabling consistenttransfer.

As described above, according to the present invention, in the case ofmelt transfer by use of a nonvolatile liquid developer, toner particlesare caused to enter the liquid-toner-softened condition, which is mostsuited for transfer of the toner image onto the printing medium. As aresult, there can be reliably maintained the condition for enablingvirtually 100% transfer of the toner image onto the printing medium;i.e., the relation “F2 (cohesive force of toner image particles)>F3(adhesive force for adhesion of toner image to printing medium)>F1(toner-image-holding force of image-bearing member).” Also, since thereis no need to apply an excessively high pressure at the time oftransfer, there can be provided a liquid-development electrophotographicapparatus that enables transfer with high image quality withoutoccurrence of an image noise, such as a shock mark.

1. A liquid-development electrophotographic apparatus in which a tonerimage formed by developing a formed electrostatic latent image by use ofa nonvolatile liquid developer is transferred from an image-bearingmember onto a printing medium by a melt transfer process, comprising:control means for controlling a viscoelasticity of a toner image on theimage-bearing member by bonding toner particles of the toner imagetogether by means of partially melting the toner particles, so as tocause the liquid toner to enter a softened condition having a carrieragent in inter-bonded-toner-particle spacing, the control means causingthe bonded toner particles to be separated from the carrier agentwithout causing the toner particles to be melted to such an extent as tobe liquefied; and carrier-agent-removing means for removing the carrieragent from the viscoelasticity-controlled toner image, thecarrier-agent-removing means having a surface in contact with thecarrier agent caused to float by use of electric field force, andremoving the carrier agent by moving the surface in a direction oppositea moving direction of the toner image.
 2. A liquid-developmentelectrophotographic apparatus according to claim 1, wherein theviscoelasticity of the toner image is controlled such that, when adynamic viscoelasticity of the toner image is measured at a forcedvibration frequency of 1 Hz and an amplitude stress of 10 Pa, a storagemodulus falls within a range of 1.0E5 Pa to 1.0E8 Pa, and a loss modulusfalls within a range of 1.0E5 Pa to 1.0E8 Pa.
 3. A liquid-developmentelectrophotographic apparatus according to claim 1, further comprisingheating means for heating the toner image formed on the image-bearingmember, wherein the viscoelasticity of the toner image is controlled insuch a manner that the heating means heats the toner image to atemperature at which the toner image exhibits a target dynamicviscoelastic value, which is determined on the basis of a previouslymeasured relationship between heating temperature and the dynamicviscoelasticity of toner particles contained in the liquid developer tobe used.
 4. A liquid-development electrophotographic apparatus accordingto claim 3, wherein, when the toner image is heated, a temperature ofthe image-bearing member is controlled to a temperature lower than aboiling point of the carrier agent.
 5. A liquid-developmentelectrophotographic apparatus according to claim 1, wherein thecarrier-agent-removing means is provided on the image-bearing member ata position located immediately before a position of transfer onto theprinting medium; bias voltage is applied to the carrier-agent-removingmeans to thereby move charged toner particles of the toner image presenton the image-bearing body and softened by the viscoelasticity controlmeans toward the image-bearing body, to thereby cause the carrier agentto float on the charged toner particles; and the floating carrier agentis removed.
 6. A liquid-development electrophotographic apparatusaccording to claim 5, wherein the carrier-agent-removing means removesthe carrier agent in such a manner that, when the toner image is to betransferred onto the printing medium, a solid content of the toner imageis 50% to 95%.
 7. A liquid-development electrophotographic apparatusaccording to claim 1, wherein, in a transfer section where the tonerimage is transferred onto the printing medium, a pressure to be appliedbetween the image-bearing member and a backup roller is set to 0.5 MPato 4.0 MPa.
 8. A liquid-development electrophotographic apparatusaccording to claim 1, further comprising a plurality of removing meansfor removing the carrier agent each time a toner image in each of aplurality of colors for color printing is transferred onto theimage-bearing member, wherein the removing means move in the samedirection as a moving direction of the toner images on the image-bearingmember.
 9. A liquid-development electrophotographic apparatus accordingto claim 1, further comprising printing-medium-heating means forpreheating the printing medium to a temperature equal to or higher thana temperature of the image-bearing member before transfer of the tonerimage onto the printing medium.
 10. A liquid-developmentelectrophotographic apparatus according to claim 1, further comprisingmeans for applying bias voltage in such a manner that electric fieldforce acts on the toner image in such a direction as to cause the tonerimage to move toward the printing medium in the course of transfer ofthe toner image onto the printing medium.
 11. A liquid-developmentelectrophotographic apparatus according to claim 10, wherein the meansfor applying the bias voltage applies the bias voltage between theimage-bearing member and a backup roller; and the resistance of theimage-bearing member is set to 1.0E7 Ωcm to 1.0E10 Ωcm.
 12. Aliquid-development electrophotographic apparatus according to claim 1,wherein a rubber material is used to form an outermost surface of theimage-bearing member from which the toner image is transferred onto theprinting medium.